1. Field of the Invention
The present invention relates to control circuits and, more particularly, to control circuits for motors and, still more particularly, such control circuits for a dual directional direct current (DC) motor employing a single alternating current (AC) power source.
2. Background Information
Alternate power sources are provided for any number of applications, which cannot withstand a lengthy interruption in electric power. Typically, power is provided from a primary source with back-up power provided by a secondary source. Often, the primary source is a utility and the secondary source is an auxiliary power source, such as an engine driven generator or a second utility source. The transfers between the two power sources can be made automatically or manually.
In the case of a generator driven auxiliary power source, power must be stabilized before the transfer can be made to the secondary source. In any event, the two power sources cannot be connected to the load simultaneously unless they are in phase. Thus, an open transition transfer may be employed in which the previously connected source is disconnected from the load before the other source is connected.
Transfer switches are known in the art. Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. Applications for transfer switches include stand-by applications, among others, in which the auxiliary power supply stands-by if the normal power supply should fail. Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls connection of the utility lines and the diesel generator to the facility load buses. In many installations, the transfer switch automatically starts the standby generator and connects it to the load bus upon loss of utility power, and reconnects the utility power to the load bus if utility power is reestablished.
Some transfer switches affect an open transition between the power sources, that is, one is disconnected from the load bus before the other one is connected. Other transfer switches provide a closed transition wherein the oncoming source is connected to the load bus before the other is disconnected, in order that both power sources are connected in parallel during the transition.
Transfer switches commonly used to connect alternate power sources to a load, including networks, utilize a pair of switches each connecting one of the sources to the load. In order to prevent connecting unsynchronized sources together, the operation of the two switches is coordinated, typically by a mechanical interlock, in order that only one switch at a time can be turned on. In many instances, it is desirable to operate the transfer switch remotely. Typically, electric motors have been used to operate the interlocks on transfer switches. See, for example, U.S. Pat. Nos. 5,081,367; 4,760,278; and 4,398,097.
A transfer switch typically comprises a pair of circuit interrupters combined with a drive input and a linkage system. The preferred types of circuit interrupters have been molded-case switches and molded-case circuit breakers because these types are commercially available in a wide array of sizes and are relatively economical compared to other options. The preferred type of drive input depends on the application for the transfer switch. Usually motors are preferred, but at other times there is a clear preference for manually-operated mechanisms.
One known automatic transfer switch employs a unidirectional motor. A motor-driven wheel rotates in one direction and employs a linkage to change the operating positions of two mounted switches (e.g., for a normal power source and an emergency power source) or two circuit breakers. The automatic transfer switch transfers between two power sources using a motor-driven arm that connects to a lever which operates both normal and emergency switches. The motor-driven lever operates in a ratchet-type operation. A rotational motion is created on an indicator wheel by the ratchet's operation.
There is room for improvement in control circuits for operating a dual directional direct current motor from a single alternating current power source.